Pair bonding is ancient - sexual monogamy is a very recent development. Cutting edge anthropology now recognizes that "co-operative breeding" was an evolutionary strategy that allowed our species to thrive. Without it, we might well have died out.
“A female who mates with several different males will have more genetically diverse offspring, boosting the chances that at least some of them will thrive.”
This goes for human females as well as other non-human animals and it is confirmed in our anatomy. The coronal ridge of the human penis is specifically designed to displace semen left there by another man or men. In addition, animals that engage in mate competition prior to copulation (like gorillas) tend to have small testes and penises. Animals that instead engage in sperm competition are more well endowed relative to body size (like chimpanzees and humans) because they need to have a large supply of semen on hand to inseminate multiple partners.
As John Odling-Smee points out in his review of noted sociobiologist Sarah Blaffer Hrdy’s book Mothers and Others, “Homo sapiens could never have evolved if human mothers had been required to raise their offspring on their own. Human infants are too helpless and too expensive in their demands for care and resources. So human females have to line up helpers — sometimes extending beyond their own kin — to raise their young. That requires both males and females to invest heavily in social skills for bargaining with other members of their groups. Hrdy suggests that females in ancestral hunting and gathering groups may have thrived because they were free to be flexible in this way.
Female flexibility was reduced when humans established settlements requiring male coalitions to defend them, probably leading to greater control of females by males… The most refreshing aspect of [this] book is the challenge [it] offers to what we thought we already knew.”
